An IGBT device has a highly complex fine geometry structure that effectively limits the size of a single IGBT chip which can be processed with an acceptable yield. Where an IGBT device of larger size is required it may be produced from an assembly of chips encapsulated in a single unit. In the main, such units have been produced in a single-side-cooled arrangement where the chips are mounted on a thermally and electrically conducting base plate or stud constituting an anode terminal and cooling contact. The emitter and gate leads are connected in common to their respective electrodes in a header seal. In order to avoid undesirable interaction between the separate chips, the connection from the common gate point to each individual chip characteristically includes a ballast resistance in its path. The inductive loop between emitter and gate leads is also desirably kept to a minimum.
To maximise the useful current rating of silicon power devices, especially diodes, thyristors and gate turn-off thyristors and to provide efficient cooling together with greater flexibility of application it has become common practice to provide disc-like encapsulations (sometimes known as the "hockey-puck" style) which may be cooled from either or both of the pole-pieces constituting the main current electrodes. The structure of such a device encapsulation is described in "Thyristor Design and Realization" by P. D. Taylor (Wiley, 1987)--see p.208 and FIG. 6.7 of the 1992 paperback edition. There are similar advantages to the user when IGBT devices are encapsulated likewise. EP-A-0 702 406 shows such a device incorporating a plurality of IGBT chips. However the internal connections shown therein, particularly those relating to distribution of the gate signal, are complex and imply potential difficulties for manufacturers seeking to offer devices in this form. There may also be mentioned as prior art EP-A- 0 773 585 and EP-A- 0 746 023.